Fiber optic telecommunications are continually subject to demand for increased bandwidth. One way that bandwidth expansion has been accomplished is through dense wavelength division multiplexing (DWDM) wherein multiple separate data streams exist concurrently in a single optical fiber, with modulation of each data stream occurring on a different channel. Each data stream is modulated onto the output beam of a corresponding semiconductor transmitter laser operating at a specific channel wavelength, and the modulated outputs from the semiconductor lasers are combined onto a single fiber for transmission in their respective channels. The International Telecommunications Union (ITU) presently requires channel separations of approximately 0.4 nanometers, or about 50 GHz. This channel separation allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Improvements in fiber technology together with the ever-increasing demand for greater bandwidth will likely result in smaller channel separation in the future.
The drive towards greater bandwidth has led to use of precision wavelength-specific DWDM devices that require careful adjustment and calibration according to the narrow transmission channel spacings. Continuously tunable lasers have been developed to aid in the test and measurement of these sophisticated devices. Tunable lasers of this sort typically utilize a tuning element, such as a pivotally adjustable grating within an external cavity, in order to generate an adjustable wavelength sweep in the laser output that can be used in the characterization of precision WDM components.
One problem which can arise with such tunable lasers is “mode-hopping” wherein the laser changes frequency discontinuously to a different longitudinal mode. When used as a telecommunication transmitter, these mode hops will cause transmission errors in the modulated data stream. One approach taken for designing tunable lasers is to use a relatively long cavity with finely spaced modes, and tune the laser in a “quasi-continuous” manner, mode hopping between the finely spaced modes. Inherent to this design is the fact that a mode hop occurs when the wavelength of operation is changed. The modes themselves are not controlled, and thus the wavelength of operation cannot be precisely tuned within the interval of a mode hop.
Another approach to tunable lasers is “mode hop free” tuning where the cavity length is synchronously changed with the wavelength tuning mechanism to keep the laser operating in the same longitudinal mode while tuning. This approach can potentially avoid the mode hopping problem and can tune to all wavelength but the constraint of synchronous adjustment makes the implementation of this design difficult.
The increasing use of re-configurable optical network architectures has recently led to the use of tunable external cavity lasers as optical communication transmitters. The tunable external cavity lasers used for telecommunication have been configured in generally the same manners as the tunable lasers used for test and measurement purposes, with the tuning of the grating being coupled to tuning of the external cavity to provide mode hop-free tuning in the laser wavelength output.
This type of mode hop-free tuning, however, has proved to be non-optimal for stable tuning to narrowly spaced transmission bands, as is required for high bandwidth DWDM systems. Mode hop-free tuning is often difficult to implement and in many cases involves a time-consuming location of a specific or “magic” pivot point to provide a specific rotation/translation relationship for tuning the grating and optical cavity length.
More particularly, the design goal of mode hop free tuning is to couple the wavelength selection filter, which selects one of the many possible longitudinal modes of the laser, with changes in the effective cavity length of the cavity, which determines the wavelength of the longitudinal modes, and thus the exact wavelength of operation. When the coupling between the cavity length and wavelength filtering is not precise, mode hops may still occur along the tuning curve, albeit at fewer points than if there was no coupling. More particularly, the coupling of the wavelength filter to the cavity modes precludes adjusting the wavelength filter without coupling this change directly into the cavity length and wavelength of operation. Although a mode hop free tuning architecture may be a good design choice for a test and measurement laser, it is not necessarily an optimal design choice for a tunable DWDM transmitter source.
When tuned, a DWDM transmitter must cease emission at the original wavelength of operation and then recommence emission at a second, precisely defined, wavelength. In general, emission at any other wavelength cannot be sent into the system since this may cause crosstalk between the channel being tuned and other transmission channels. One method for tuning is to mode hop the laser directly to the target wavelength. It is more practical to tune with the laser powered down, shuttered, filtered, or otherwise guaranteed to avoid other wavelengths during tuning, where these precautions may be taken at the system level or at the source. With these precautions, the precise mode hop behavior of the laser during tuning between channels is not important. It is important that the target wavelength is precisely achieved. The effective cavity length, which determines the exact wavelength of operation, must be precisely controlled. Only after tuning and transmission into the system commences must mode hopping, which causes amplitude and frequency changes in the source, be avoided.
In some DWDM systems, the channel frequencies are adjusted during operation to maximize system capacity at acceptable bit error rate. Telecommunication sources must operate over a large temperature range, with −5° C. to 70° C. being typical, and the effect this temperature has on effective cavity length, filter characteristics, and the general state of the laser, must be countered. The operation characteristics of thermal compensation and fine frequency control are similar and lead to the requirement that a telecommunication transmitter have precise control of cavity length and the wavelength filtering element. In view of these design considerations, a mode hop free tuning approach does not confer the same advantages in DWDM source applications as it does in a test and measurement application. There is a need for a tunable laser source for DWDM applications which has independent fine control of the wavelength filtering element and cavity length but is not restricted to avoiding mode hop behavior when coarsely tuning.